Simple_vegetation_model

[mp586]

Simple vegetation model

1. Small additions to the bucket model to allow the user to set a ``vegetation pre-factor'' between 0. and 1. (where 1. is the original bucket model), to mimick stomatal closure with higher CO2 levels. 0 means no evaporation from the land surface.

The namelist option for setting the veg_evap_prefactor to e.g. 0.5 is :

'surface_flux_nml': {
    'veg_evap_prefactor': 0.5 }

2. I've added the option to output ''potential evapotranspiration'' (see e.g. Budyko, 1974 or Milly and Dunne, 2016), defined here as the evaporation that would occur if the bucket were full.

diag.add_field('atmosphere', 'potential_evap', time_avg=True)

3. I did some code-cleanup in surface_flux.F90 and idealized_moist_phys.F90, getting rid of the author initials in the comments

Budyko, M. I. (1974). Climate and Life. Academic Press, New York and London. English ed. edited by David H. Miller. ISBN 0-12-139450-6.
Milly, P. C. D. and Dunne, K. A. (2016). Potential evapotranspiration and continen- tal drying. Nature Climate Change, 6:946 – 949. doi: 10.1038/nclimate3046.

The trip tests (excluding the socrates-related ones) passed fine for those changes

Results for all of the test cases ran comparing 9cfd43e and 74f12d0 are as follows...
axisymmetric, : pass
bucket_model, : pass
frierson, : pass
giant_planet, : pass
held_suarez, : pass
MiMA, : pass
realistic_continents_fixed_sst, : pass
realistic_continents_variable_qflux, : pass
top_down_test, : pass
variable_co2_grey, : pass
variable_co2_rrtm : pass
Congratulations, all tests have passed